Communication device, communication system and communication method

Abstract

A communication device according to the present invention aims to restart data transmission/reception between communication devices in a short amount of time, without performing key sharing again, even when a communication device of a transmitting side is rebooted. The communication device includes a volatile memory storing a count value, a generator generating data including a count value, a communicator transmitting data to another communication device, a storage instructor, each time the communicator transmits data, updating the volatile memory with a count value, and a nonvolatile memory. A count value is incremented each time the communicator transmits data, the storage instructor causes the nonvolatile memory to store a count value at certain intervals, and the generator, when the communication device is rebooted, includes in data a sum of a certain value and a count value stored in the nonvolatile memory.

The invention claimed is: 
     
       1. A communication device that transmits a data frame to another communication device, comprising:
 a first memory that is a volatile memory, storing a count value that is utilized by the other communication device to determine whether or not a data frame that the other communication device has received and a data frame that the communication device has transmitted equal each other; 
 a frame generator that reads out a count value currently stored in the first memory and generates a transmission data frame including a count value that is generated based on the count value currently stored in the first memory; 
 a communicator that transmits the transmission data frame to the other communication device; 
 a storage instructor that updates the count value currently stored in the first memory by causing the first memory to store the count value stored in the transmission data frame when the communicator transmits the transmission data frame; and 
 a second memory that is a nonvolatile memory, wherein 
 the count value stored in the transmission data frame is generated by incrementing the count value currently stored in the first memory when the communicator transmits the transmission data frame, 
 the storage instructor causes the second memory to store the count value stored in the transmission data frame at a certain interval, 
 when the communication device is rebooted, the frame generator reads out a count value currently stored in the second memory and stores to the transmission data frame, as the count value, a sum of the count value currently stored in the second memory and a predetermined value, and 
 the predetermined value is calculated from the certain interval and a frequency at which the communicator performs data frame transmission. 
 
     
     
       2. The communication device according to  claim 1 , wherein
 the certain interval is an interval from storing of the count value currently stored in the second memory to when the count value stored in the transmission data frame becomes greater than the count value currently stored in the second memory by a certain value. 
 
     
     
       3. The communication device according to  claim 1 , wherein
 the certain interval is an interval from storing of the count value currently stored in the second memory to when the communicator has transmitted the transmission data frame a certain number of times. 
 
     
     
       4. The communication device according to  claim 1 , wherein
 the certain interval is a predetermined amount of time. 
 
     
     
       5. The communication device according to  claim 1 , wherein
 the predetermined value is, instead of being a value calculated from the frequency and the certain interval, an increase in the count value stored in the transmission data frame during the certain interval. 
 
     
     
       6. The communication device according to  claim 1 , wherein
 the predetermined value is a product of the frequency and the certain interval. 
 
     
     
       7. The communication device according to  claim 1 , wherein
 the predetermined value is, instead of being a value calculated from the frequency and the certain interval, a value smaller than an increase in the count value stored in the transmission data frame during the certain interval. 
 
     
     
       8. The communication device according to  claim 7 , wherein
 when the transmission data frame is not authenticated by the other communication device, a value larger than the predetermined value is added to the count value read out from the second memory. 
 
     
     
       9. The communication device according to  claim 1 , further comprising:
 a storage interval receiver that receives an instruction related to the certain interval. 
 
     
     
       10. A communication method of a data frame performed by a communication device that transmits a data frame to another communication device, comprising:
 storing in a first memory that is a volatile memory, a count value that is utilized by the other communication device to determine whether or not a data frame that the other communication device has received and a data frame that the communication device has transmitted equal each other; 
 reading out a count value currently stored in the first memory and generating a transmission data frame including a count value that is generated based on the count value currently stored in the first memory; 
 transmitting the transmission data frame to the other communication device; 
 updating the count value currently stored in the first memory by causing the first memory to store the count value stored in the transmission data frame when the transmitting transmits the transmission data frame; 
 generating the count value stored in the transmission data frame by incrementing the count value currently stored in the first memory when the transmitting transmits the transmission data frame; 
 causing the second memory to store the count value stored in the transmission data frame at a certain interval; and 
 when the communication device is rebooted, reading out a count value currently stored in the second memory and storing to the transmission data frame, as the count value, a sum of the count value currently stored in the second memory and a predetermined value, wherein 
 the predetermined value is calculated from the certain interval and a frequency at which the transmitting performs data frame transmission. 
 
     
     
       11. A communication system that transmits a data frame from a first communication device to a second communication device, wherein
 the first communication device comprises
 a first memory that is a volatile memory, storing a count value that is utilized by the second communication device to determine whether or not a data frame that the second communication device has received and a data frame that the first communication device has transmitted equal each other, 
 a frame generator that reads out a count value currently stored in the first memory and generates a transmission data frame including a count value that is generated based on the count value currently stored in the first memory, 
 a communicator that transmits the transmission data frame to the second communication device, 
 a storage instructor that updates the count value currently stored in the first memory by causing the first memory to store the count value stored in the transmission data frame when the communicator transmits the transmission data frame, and 
 a second memory that is a nonvolatile memory, wherein 
 when the first communication device is rebooted, the frame generator reads out a count value currently stored in the second memory and stores to the transmission data frame, as the count value, a sum of the count value currently stored in the second memory and a predetermined value, 
 
 the second communication device receives a data frame transmitted from the first communication device, and authenticates whether a data frame that the second communication device has received and a data frame that the first communication device has transmitted equal each other, and 
 the predetermined value is calculated from the certain interval and a frequency at which the communicator performs data frame transmission.